Various structures for radiating heat generated by electronic components have been proposed with respect to circuit boards having electronic components mounted, and a wiring pattern formed, on the upper surface of an insulating layer.
For example, according to circuit boards of JP 2007-36050 A, JP 2014-179416 A, JP 2014-179415 A, JP 2012-119607 A, JP 2006-49887 A, JP 2008-251671 A, JP 2014-157949 A, and JP 6 (1994)-244303 A, a thermal conductor made of a metal such as copper is embedded in an insulating layer, and an electronic component that generates heat is mounted above the thermal conductor.
According to JP 2007-36050 A and JP 2008-251671 A, a conductor such as a land or a solder is provided on the upper surface of the thermal conductor, and an electronic component is mounted on the conductor. According to JP 2014-179416 A, JP 2014-179415 A, JP 2014-157949 A, and JP 6 (1994)-244303 A, an insulating layer is provided on the upper surface of the thermal conductor, and an electronic component is mounted on the upper surface of the insulating layer via a conductor. In JP 2014-179416 A, JP 2014-179415 A, and JP 2014-157949 A, the material of the insulating layer on the upper surface of the thermal conductor and the material of an insulating layer around (on the lateral side) of the thermal conductor are the same. Also, according to JP 2014-179416 A and JP 2014-179415 A, the insulating layer has a high thermal conductivity. According to JP 6 (1994)-244303 A, the insulating layer on the upper surface of the thermal conductor is thermally enhanced and reinforced by using, as the material of the insulating layer, glass or polymer fiber reinforcement impregnated with a high thermal conductivity additive (zinc oxide, aluminum oxide, aluminum nitride, etc.). According to JP 2012-119607 A and JP 2006-49887 A, the main body of the electronic component (the package portion of a semiconductor) is mounted on the upper surface of the thermal conductor via a thermal conductive member. Also, the thermal conductive member of JP 2006-49887 A has insulation properties.
Moreover, according to JP 2007-36050 A, an insulating layer is provided on the lower surface of the thermal conductor, and a heat radiator is attached on the lower surface of the insulating layer. According to JP 2014-179416 A, JP 2014-179415 A, JP 2012-119607 A, and JP 6 (1994)-244303 A, the lower surface of the thermal conductor is exposed from the insulating layer. Also, JP 2014-179416 A discloses an example where a conductor or an insulating layer with high thermal conductivity is provided on the lower surface of the thermal conductor and the lower surface of the insulating layer around the thermal conductor. JP 2012-119607 A discloses an example where a heat radiator is screwed to the lower surface of the thermal conductor.
According to JP 2006-49887 A, a copper layer is provided on the lower surface of the insulating layer around the thermal conductor, the copper layer and the thermal conductor are connected, and the lower surface of the thermal conductor is exposed from the copper layer. Also, JP 2006-49887 A discloses an example where a heat radiator is provided on the lower surface of the thermal conductor and the lower surface of the copper layer via an insulating layer with high thermal conductivity. According to JP 2008-251671 A and JP 2014-157949 A, an insulating layer is provided on the lower surface of the thermal conductor and the lower surface of the insulating layer around the thermal conductor, and a metal layer for heat radiation is provided on the lower surface of the insulating layer. Moreover, according to JP 2008-251671 A, the insulating layer that is in contact with the metal layer has a high thermal conductivity.
In a case where a thermal conductor is embedded in an insulating layer of a circuit board, and an electronic component is mounted on the upper surface of the thermal conductor, as in a conventional case, heat that is generated by the electronic component is directly transferred to the thermal conductor, and is radiated below the thermal conductor. However, since the electronic component and the thermal conductor are electrically connected, it is difficult to mount other electronic components in the vicinity. Moreover, there are problems that it is hard to form an electrical circuit on the circuit board, and that the mounting density of the circuit board is reduced.
On the other hand, in the case where an electronic component is mounted on the upper surface of the thermal conductor via the insulating layer, heat generated by the electronic component is not easily transferred to the thermal conductor, and the efficiency of heat radiation to below the thermal conductor is reduced.
Furthermore, if the insulating layer is formed using a material impregnated with a high thermal conductive additive, the thermal conductivity of the insulating layer is increased, but the hardness of the insulating layer is also increased due to the mixing of the additive. Accordingly, for example, even if a through hole is to be provided to a multi-layer circuit board where wiring patterns are provided on the upper and lower surfaces and inside the insulating layer in order to connect the plurality of wiring patterns at different layers for the purpose of increasing the degree of freedom regarding the circuit configuration, it is difficult to form the through hole in such away as to penetrate the hard insulating layer.